1. Field of the Invention
This invention relates to a system for controlling a heater mounted on a spark plug igniting the air-fuel mixture introduced into the combustion chamber of an internal combustion engine, and more particularly to such a system for use in an internal combustion engine which uses a hybrid fuel consisting of a blend of gasoline and alcohol.
2. Description of the Prior Art
In a gasoline engine, gasoline (the fuel) is introduced into the combustion chambers together with air in the form of an air-fuel mixture. When the air-fuel mixture in a combustion chamber is ignited it burns explosively. For igniting the air-fuel mixture, there is generally used a spark plug which produces an electric spark. The spark plug has a pair of electrical discharge electrodes disposed across a gap of prescribed magnitude. A high voltage is applied across the electrodes to produce a spark discharge between them. The air-fuel mixture in the combustion chamber is ignited by the spark accompanying the discharge.
In a system using a spark plug of this type, since the spark producing portion of the spark plug (the discharge electrode portion) is disposed in the combustion chamber, a phenomenon known as "carbon fouling" occurs when the spark plug temperature is too low. Namely, the spark producing portion may be fouled with carbon and other products of incomplete combustion. When this happens, electricity leaks through the adhering carbon, reducing the voltage between the discharge electrodes and weakening the spark.
For avoiding this problem, it has been proposed, in Japanese Laid-Open Utility Model Publication No. 60(1985)-42291, for example, that a heater be mounted in the vicinity of the spark producing portion of the spark plug and be turned on to heat the spark plug when the engine is cold. In a system equipped with an auxiliary low-temperature starting device of this type, the heating of the spark plug ensures that any carbon adhering to the spark producing portion will be burned off. In other words, carbon and other fouling materials will be removed by a self-cleaning effect. Similar technique has also been proposed in Japanese Patent Publication No. 49(1974)-8651.
On the other hand, an increasing number of engines which use a hybrid fuel consisting of a blend of gasoline and alcohol are being put into use nowadays, mainly with the aim of reducing gasoline consumption. Since alcohol has a higher boiling point and larger latent heat of vaporization than gasoline, in such an engine the fuel tends to form droplets in the combustion chambers. Therefore, during a cold engine start, liquid alcohol is apt to adhere between the spark plug electrodes. As this reduces the electrical insulation between the electrodes, it may become impossible to develop a high enough discharge voltage to produce a spark.
If the engine uses the heater-equipped spark plug just referred to, the heating of the discharge electrode portion when the heater is turned on will vaporize any alcohol adhering between the electrodes and thus ensure reliable production of a discharge spark.
A problem arises, however, owing to the fact that when a gasoline-alcohol blend fuel is used and supplied to the combustion chamber, the alcohol concentration of the blended fuel does not stay constant. Since the amount of liquid alcohol adhering to the spark plug discharge electrodes increases with increasing alcohol concentration of the fuel, the heating of the spark plug is able to reliably prevent adherence of liquid alcohol only if the heating temperature is set high enough to vaporize the adhering liquid alcohol when the alcohol concentration is at its highest. The heater thus has to raise the spark plug to a considerably high temperature. This temperature is higher than the temperature for burning off carbon adhering to the electrodes.
Heating the spark plug to the required temperature accelerates electrode consumption. It also increases the risk of plug burnout. The service life of the spark plug is therefore reduced. In addition, as a larger amount of power has to be supplied to the heater, power consumption is increased.